This invention relates to a method by which comparatively low-voltage electric wires and cables laid at power stations and other locations can be diagnosed for deterioration of their polymer-insulation in a non-destructive way. The invention also relates to a measuring apparatus used for implementing that method.
Many kinds of electric wires and cables are installed in nuclear power plants and they are more or less exposed to a radiation and/or thermal atmosphere. Including those which are used in such environments, electric wires and cables generally experience electrical or mechanical deterioration during prolonged use on account of various factors and must eventually be replaced.
The principal reason for the deterioration of polymer-insulators (consist of mainly a sheath and a insulator) in electric wires and cables would be oxidation and the resulting hardening (or softening in a rare case) causes a drop in the mechanical or electrical characteristics of electric wires and cables to such an extent that they will no longer perform in a normal and safe way. Hence, in order to enhance the safety of power plants and other facilities where electric wires and cables are installed so as to prevent the occurrence of accidents, it is very important that any deterioration of the electric wires and cables used be detected by a simple and yet reliable method. Accordingly, the development of such a method has been strongly desired.
In response to this need, active R&D efforts have long been made on a so-called "live-line diagnostic method" which is a non-destructive method for checking any deterioration in the polymer-insulation of live high-voltage cables. In addition to the already popular insulation resistance, dielectric constant and d.c. leakage current methods, various other techniques have recently been proposed, including a reverse absorption current method, a residual voltage method and a potential attenuation method. Today, these electrical non-destructive diagnostic methods also have been studied to make it feasible to check the deterioration of electric wires and cables with fairly high reliability.
High-voltage cables have a shield layer around the cable structure which can be utilized to perform diagnosis for deterioration by non-destructive electrical methods. Thus, the conventional methods for checking the degree of deterioration in high-voltage cables are not suitable for application to comparatively low-voltage wires and cables which do not have a shield layer.
Under these circumstances, the deterioration of low-voltage electric wires and cables has been checked by skilled personnel through visual inspection or by destroying removed parts so as to measure the loss in the electrical and mechanical characteristics, in particular, the residual elongation of their polymer-insulation materials. The visual inspection method is most commonly used to check the degree of deterioration of the polymer-insulation in low-voltage electric wires and cables which account for nearly half of the wires and cables used in power plants but this technique requires considerable skill on the part of inspectors and, furthermore, even skilled inspectors are subject to personal errors and will experience great difficulties in accomplishing accurate and reliable checking of the deterioration of the polymer-insulation. The second approach which involves destruction of removed parts not only lacks convenience but also requires substantial labor and time.